Hydraulic pile-driving device



Sheet 1 of 5 Filed May 24, 1967 March 11, 1969 J. w. JANSZ HYDRAULIC FILE-DRIVING DEVICE Sheet Filed May 24, 1967 March 11, 1969 J. W, JANSZ HYDRAULIC FILE-DRIVING DEVICE Sheet Filed May 24, 1967 o a a United States Patent 6607149 US. Cl. 173-134 9 ClalmS Int. Cl. E02d 7/10; B25d 9/12 ABSTRACT OF THE DISCLOSURE A hydraulic pile-driving device includes at least one hydraulic cylinder which is connected with a ram body to move it up and down, a hydraulic main circuit operating the hydraulic cylinder being controlled by a hydraulic control circuit operating in dependence upon the position of the piston in the hydraulic ram cylinder. Special hydraulic circuit connections and components prevent hydraulic shocks and maintain the downward acceleration of the ram at a constant value.

Background of the invention Field of the invenri0r .The invention relates to improvements in hydraulic pile-driving devices.

Prior art.A known hydraulic pile-driving device comprises a cylinder with piston connected with the ram body and adapted to move this upwardly and downwardly by means of the alternating pressures on either side of the piston for the control of which a hydraulic circuit has been provided with a control slide between the cylinder and the source of fluid pressure, this control slide enabling the supply of high pressure to be directed to one side or the other side of the piston at the same time connecting that side of the piston opposite the high pressure supply side with a discharge, the control Slide being controlled by the movements of the piston with respect to the cylinder.

With such known device the switching at the end of the return stroke takes place mechanically in that a cam follower connected with the piston engages acam which acts upon the control slide. At the end? of'. the working stroke, however, the control slide is switched by means of the pressure impact generated in the hydraulic circuit when the ram hits the pile. With the known devices the switching of the control slide by means of the impact in the hydraulic circuit is considered an improvement over switching the control slide at the end of the working stroke by means of a cam and cam follower in the same way as this still is done at the end of the return stroke. In piledriving devices the mechanical means for operating the control slide are not able to stand up against the very high mechanical loads occurring in use. However, the impacts used in the known devices have very different values since this impact depends upon the resistance the ram meets in performing its stroke which resistance depends upon t-he condition of the soil and may vary in a broad range. In the known devices even if there would occur no impact at all still the pressure supply from the pump would be SllffiClCl'lt to assure that switching takes place. This, however, means that the hydraulic system especially the switching member has to respond to very low and very high pressure changes and it is very difiicult to obtain a switching member which operates reliably in such a broad pressure range.

With hydraulic pile-driving devices it is desirable that during sequential working strokes the magnitude of the acceleration in general has always the same value. This acceleration as a rule is greater than the acceleration of the gravity to gain time and energy. The ram will rebound more or less strongly depending upon the resistance and this interferes with the accumulation of energy.

in a high pressure accumulator necessary in the known device. Accordingly this interferes as well with the acceleration occuring during the working stroke. This means that with the known devices this acceleration cannot, at least not without difiiculty, be maintained at an almost constant value.

Summary of the invention This invention provides a pile-driving device which needs no mechanical means for its control and does not make use of pressure impacts occurring in the hydraulic system, a further acceleration during each stroke has a constant value which is not affected by the resistance of the soil.

A further purpose of the invention is to make use in a more efficient way of the impact energy of the ram upon the pile.

According to the invention this is achieved in that for the control of the control slide by means of the piston a hydraulic control circuit has been provided which on one side has been connected with hydraulic actuators of the control slide and on the other side with the source of liquid under pressure and comprises a pilot slide controlling the connection of the control slide actuators with the pressure source or with the discharge respectively and which has actuators itself which are connected to a cylinder port controlled by the piston in advance of the end of the down-ward stroke, which last mentioned connection has been provided with a blocking slide with actuators which in dependence of the position of the control slide are under the pressure of the supply and/ or the discharge of the main circuit while further control conduit is controlled by the piston at the end of its return stroke, which control conduit governs the pressure in a connection of the blocking slide with the pressure source, in which connection there is a throttle, the blocking slide controlling both conduits which are controlled by the piston such, that in the two different positions of the slide one of the connections is always connected with the actuator of the pilot slide and the other is blocked.

According to the invention the reversal of the direction of movement of the ram at *both ends of the strokes is performed by a hydraulic control system operating in dependence of the position of the piston within its cylinder and independent of the impact generated by the ram. The reversal of the direction of movement of the cylinder at the end of the working stroke is performed in that the high pressure of the main circuit existing above the downwardly moving piston moves into the control circuit and acts upon the actuator of the pilot slide of the control slide which then is shifted. This takes place as soon as the downwardly moving piston passes the cylinder port adjacent to the lower end of the cylinder.

It is observed in this respect that the distance of said port from the lower end is not very critical. The main requirement is that the piston actually passes said port. There is no risk that the piston will hit the lower end of the cylinder since the piston rod is connected to the ram and the ram is guided in a housing supported upon the pile.

If the distance of the port from the bottom of the cylinder would be so great that switching would take place before the end of the working stroke, so that a counter pressure would be generated below the cylinder before the ram hits the pile this does not affect the operation since said counter pressure is acting on the housing and accordingly also upon the head of the pile, This is even advantageous in that it prevents jumping of the housing. The reversal of the movement at the end of the return stroke according to the invention can be performed in that the control conduit which at the end of the return stroke is controlled by the piston, opens into a chamber encircling the piston rod which is selectively connected with the adjacent cylinder space by means of a reduced diameter constriction of the piston rod or by means of a similar connection, which control conduit has been connected with the connection between the blocking slide and the pressure source at a place located between the throttle and the blocking slide. In this way the connection of the blocking slide with the control circuit can be a high pressure connection necessary for taking over the high pressure for the actuator of the pilot slide when the blocking slide has been switched during the return stroke while a further pressure drop can take place in the control conduit as soon as the constriction of the piston rod has connected the space around the piston rod with the cylinder space due to which a return of the pilot valve automatically takes place so that the control valve is moved into position for the next working stroke.

Instead of making use of a pressure drop in the space above the piston during the return stroke it is also possible, in a corresponding manner as done at the end of the working stroke to perform the reversal of movement at the end of the return stroke by means of the pressure existing below the piston. According to the invention this is achieved in that the control conduit controlled by the piston at the end of its return stroke is formed 'by a conduit between a cylinder port spaced from the upper end of the'cylinder and the actuator of an additional slide located in a branch of the connection between the pressure source and the blocking slide and branching from it at a place between the throttle and the blocking slide, which additional slide may selectively connect or close the connection with the discharge.

With a hydraulic pile-driving device having a plurality of cylinders and pistons for actuating the ram it is possible to form the reduced diameter constrictions of the piston rods, provided for the control of the control circuit at the end of the return stroke, with different lengths and to include a switching member by means of which at choice one of said cylinders may be connected to the blocking slide controlling the operation of the pilot slide. Due to this, it is possible to operate with dilferent strokes.

If the ram rebounds strongly this may require such a great amount of liquid below the piston that said quantity cannot be delivered by the pump immediately. In a device in which the return conduit towards the discharge has been provided with a low pressure accumulator this can be achieved according to the invention in that the low pressure accumulator has been connected through a one way valve with the conduit of the main circuit, which after switching of the control slide at the end of the working stroke becomes the supply conduit for liquid under pressure. If there is high pressure in said conduit the one way valve will take care that the low pressure accumulator is closed with respect to said conduit. If, however, as the result of the rebounding an extraordinary quantity of liquid is required in said conduit, so that notwithstanding its connection with the high pressure source a low pressure occurs in said conduit, then the liquid from the low pressure accumulator can flow through the one way valve into the conduit to fill up the gap. In this way a temporary vacuum in the conduit can be avoided.

Further it may be desired to brake the recoiling piston as quick as possible and according to the invention this may be achieved in that the conduit of themain circuit which during the working stroke is under high pressure, has been provided between the control slide and a high pressure accumulator with a one way valve and an overpressure valve connected in parallel with the one way valve, which overpressure valve by means of the control conduit has been connected with said conduit of the main circuit in which there is low pressure during the working stroke and high pressure during the return stroke the overpressure value being made such that it forms a free passage with high pressure in its control conduit and acts as an overpressure valve with low pressure in said control conduit.

With hydraulic pile-driving devices it is well known to have a more or less resilient member between the ram and the pile head. This is done to prevent the impact acting upon the pile from passing a dangerous value which could destroy the pile. Such a resilient member shows a timeimpact diagram according to which an important part of the impact energy is lost before a force is created of sufiicient value to overcome the resistance of the soil.

According to the invention a more eflicient use is made of the impact by using a resilient member which in the direction of the stroke has been prestressed. This has the result that at the impact there almost immediately is suflicient force available and remains available almost up to the end of .the impact to overcome the resistance of the soil. Due to this a much better time-impact diagram is obtained.

In a preferred embodiment this prestressed resilient member comprises a body vertically movable within a cylinder in the lower end of the ram, which body is closed at the bottom and open at the top, a piston is guided in a fluid tight way in a vertical direction in the body, the space below this piston within the body is filled with a gas under pressure while the space above the piston has a con nection with the hydraulic circuit through a conduit with a one way valve, the conduit extending through the piston rod connected to the ram and opening into one of the cylinders for operating the device and having a connection with the cylinder space above the piston of said ram cylinder.

Brief description of the drawings The invention now will be further elucidated with reference to the accompanying drawings.

FIG. 1 is a schematic cross section through a device according to the invention.

FIG. 2 is a hydraulic circuit diagram for the elucidation of the operation of the device according to the invention.

FIG. 3 is a hydraulic circuit diagram of a modified form of the device according to the invention.

FIGS. 4 and 5 are time-impact graphs showing the difference between using a resilient member between the ram and the pile head and a prestressed resilient member respectively.

Description of the preferred embodiments The device shown in FIG. 1 comprises a ram 1 having at its upper end a yoke 2 connected with two piston rods 3 and 4 respectively said piston rods are provided with pistons 7 as well as rods 8 for guiding the pistons. The ram 1 and the piston rods 3 and 4 with the pistons 7 and rods 8 are shiftably guided in a housing 5 which includes cylinder space 6 for the piston 7. These pistons 7 in cylinders 6 serve for moving the ram 1 upwardly and downwardly and to this end the cylinders 6 are provided with connections 9 and 10 respectively for the supply and discharge respectively of liquid under high pressure. These main connections 9 and 10 can be connected in turn with the supply of liquid under high pressure or with the dis charge respectively so that the movements of the pistons in both directions are controlled by the difference in pressure on the opposite sides of the pistons 7.

The hydraulic pile-driving device can be provided at the lower end of the ram 1 with a prestressed resilient member formed by hammer body 11, which like a piston is shiftably guided within a bore of the ram 1, its downward movements being limited by collar 12. In the hammer body 11 a floating piston 13 is provided, the stroke length of which is limited upwardly by collar 14. A space 15 below the floating piston can be filled with a gas under pressure through channel 16a and valve 17a, as a result of which the piston 13 is positioned with its upper side against the collar 14. During operation, space 16 above the floating piston will be filled with liquid under pressure through the conduit 17, the one way valve 18 and a central bore of the piston rod 3 opening into the cylinder space 6 above the piston 7, so that the maximum value of the liquid pressure obtainable in the cylinder space 6 can be generated in the space 16 and accordingly will press the floating piston 13 downwardly and in this way will form the resilient prestressed member.

During operation of the device the ram 1 with the hammer body 11 Will always at the end of the working stroke hit plate 19 and accordingly will transmit its impact energy towards the pile which is to be driven into the soil, the head of the pile being within a skirt 20. The combination of plate 19 and skirt 20 is guided within the housing 5 by means of the cylindrical sleeve 21 to prevent the shock between the hammer head 11 and the plate 19 during operation from being transmitted towards the housing 5.

The hydraulic circuit diagram of FIG. 2 only shows one of the cylinders 6 with piston 7 and piston rods 3 and 8 respectively of the device shown in FIG. 1, but it will be clear that in case a plurality of cylinders is used nothing will be changed in principle.

In the circuit diagram the main conduits of the hydraulic circuit are indicated with straight lines while the conduits of the control circuit are indicated with interrupted lines.

It further will be understood that with an embodiment having a plurality of simultaneously operating cylinders it will be sufficient to connect the control circuit to only one of said cylinders.

The source of liquid under pressure is provided by the pump 24 delivering liquid under high pressure through the manually operable slide valve 25, conduit 26, high pressure hose 27 and a constant pressure control valve 28 with a control conduit 72. From the control valve 28 the high pressure liquid can flow through the conduit 68 and the portion of the control slide 29 as shown, conduit 30, one way valve 31 and conduit 32 through the connection 9 into the cylinder space 6. The conduit 32 may be provided with a high pressure accumulator 63. If the above described flow of liquid under pressure takes place the piston 7 will be forced downwardly and performs its working stroke, the acceleration of which depends on the pressure level in the high pressure conduit, the pressure diflerence, the Working area of the piston and the resistances in the conduits. In pile-driving devices it is necessary to adapt said factors with respect to each other such that the piston and accordingly the ram will obtain an acceleration which is a multiple of the acceleration of gravity.

The downwardly moving piston 7 forces the liquid below it through the connection 10, conduits 33, 34 and control slide 29 back into the reservoir 37. This connection may be provided between the reservoir and the control slide 29 with a low pressure accumulator 35, a low pressure hose 26 and an overpressure valve 38.

Between the beginning of the high pressure conduit 26 and the end of the return conduit an overpressure valve 39 may be provided through which excess of liquid supplied by the pump 24 may flow back. The overpressure valve 38 may be adjusted in such a way that at the discharge opening 42 of the low pressure accumulator 35 a pressure exists which is higer than the pressure in the reservoir 37.

The circuit diagram further shows that the control slide 29 has hydraulic actuators 56 and 57 and that a pilot slide 47 has been provided with a hydraulic actuator 46 and an actuating spring 48. Further a blocking slide 45 has been provided with a hydraulic actuator 49 and an actuating spring 49. Instead of the springs 48 and 49' hydraulic actuators may be used as well.

The pilot slide 47 has a connection 52, 53 with the high pressure main conduit and a connection 60 with the low pressure main conduit between the control slide 29 and the reservoir 37. The blocking slide 45 has connections 43 and 79 with ports 22 and 23 respectively of the cylinder 6 as well as a connection with the actuator 46 of the pilot slide 47.

From FIG. 2 it further appears that the line 79 of the blocking slide is connected with the cylinder port 23 but also with the lines 7 8, 77 to the high pressure main conduit 52. In this connection a throttle 76 has been provided. The port 23 connects this line with an annular space 74 surrounding the piston rod which space is separated from the cylinder space 6. The piston rod has been provided with a constriction 75 and during the return stroke as soon as the constriction of the piston rod passes the separating wall between the annular space 74 and the cylinder space 6, space 74 will be connected with the cylinder space 6 above the piston 7 through the constriction 75.

FIG. 2 further shows a line 69 connecting the low pressure accumulator 35 through a one way valve 70 with the lower main conduit 33. It further shows that the upper main conduit 32 has been provided with a high pressure accumulator 63, a one way valve 31 and an overpressure valve 61 with spring 73 and control conduit 62.

It is observed that the low pressure accumulator, the high pressure accumulator and the assembly of one way valve 31 and overpressure valve 61 are not necessary for a desired operation but only in certain circumstances, as will be explained below, may improve the operation.

The device described operates as follows:

If, as shown, the piston 7 moves downwardly under the pressure of the liquid supplied through the connection 9, the piston shortly before the end of its working stroke will pass the cylinder port 22 of the control circuit. The cylinder port 23 of the control circuit, connecting the annular space 74 with the control circuit has been separated from the pressure space above the piston by the piston rod 3. Through the conduits 52, 77, 76 and 78, however, high pressure also exists in said space 74.

The port 22 has been connected with the actuator 46 of the pilot slide 47 through the control conduit 43, channel 44 of the blocking slide 45 and conduit 81 and this pilot slide 47 is in the position shown due to the operation of the spring 48. In this position of the pilot slide 47 high pressure is supplied to the actuator 56 of the control slide 29 through the conduits 52, 53, 54 and 55 and low pressure exists in the actuator 57 through the conduits 60, 59, and 58 so that the control slide is in the position shown.

The blocking slide 45 takes the position shown under the influence of the hydraulic actuator 49 which through conduits 68 and 50 is under the high pressure of the supplying main conduit 30 acting against the spring 49.

As soon as the piston 7 passes the port 22 high pressure through the connections 43, 44, 81 will be transmitted to the hydraulic actuator 46 so that the slide 47 is switched and takes the position shown in the lower half of the slide, as a result of which the actuator 57 of the control slide 29 through the connection 64 of the pilot slide 47 will obtain high pressure while the actuator 56 through the connection 65 of the pilot slide 47 will obtain low pressure so that the control slide 29 switches and forms the connection shown in the upper part of said slide in the drawing. Due to this high pressure will be supplied through connection 66 of the control slide 29 to the connection 10 and the connection 9 will be connected with the discharge through the connection 67 of the control slide 29.

Due to the fact that now the main conduit 30 is under low pressure, the control conduit 50 will also be under low pressure so that the spring 49 is able to move the slide 45 so that the control conduit 43 of the port 22 will be blocked and accordingly will not be influenced by the piston 7 passing this port again during the return stroke.

Through the conduits 77, 76, 78, 80 and 81 care is taken that the switching of the slide 45 has no influence upon the high pressure in the hydraulic actuator 46 of the pilot slide 47.

After performing the working stroke the ram will rebound and this means that the piston 7 will return quickly therewith assisted by the supply of high pressure liquid through the connection 10. Under extreme conditions the low pressure accumulator now 'may have a special function through the connection of the conduit 33, 69 with the space 71 of the low pressure accumulator. During fast rebounding of the piston 7 it may happen that the high pressure supply conduit is not able to supply liquid sufliciently quick to fill the space below the rebounding piston 7. As long as there is high pressure in the conduit 33 the connection with the low pressure accumulator through 69, 71 and the one way valve 70 will be closed. This rebounding and the insufiicient supply of liquid in the cylinder space below the piston 7 results, however, in a pressure drop in the conduit 33. Due to the described connection of the low pressure accumulator it now becomes possible, during this period of reduced pressure in the high pressure conduit 33, 10, to supply liquid from the low pressure accumulator through 70, 71 and 69, in which case a piston 41 in the low pressure accumulator by closing the outlet 42 will take care that the low pressure system connected with the low pressure accumulator will not change its pressure. The overpressure valve 28 also takes care that the temporarily occuring low pressure which also will exist in the conduit part 68, does not influence the pressure level in the conduits 51, 52 in front of the valve 28 controlled by the control conduit 72 such that it allows the passage of liquid but maintains the pressure in the conduit 51 in front of it at a constant value.

If extremely sharp rebounding of the ram takes place and accordingly of the piston 7 it may be required to brake such movement and to this end the overpressure valve 61 has been provided within the control circuit which has been placed in parallel with the one way valve 31 and has been provided with the control conduit 62 influenced by the pressure in the conduit 33. This overpressure valve 61 has been made such that it provides a free passage for liquid if there is high pressure in the control conduit 62 but operates as an overpressure valve in dependence of the tension of the spring 73 if there is low pressure in the control conduit 62. If sharp recoiling takes place together with the above described temporary pressure drop in the conduit 33 this pressure drop also will exist in the control conduit 62 of the valve 61 so that the one way valve 31 will close the return conduit 32 and accordingly will brake the movement of the piston 7. In this way the piston 7, notwithstanding whether it rebounds strongly or weakly, shortly after the beginning of the return stroke will return with a speed which is the same for all situations.

The switching at the end of the return stroke takes place as follows. The piston rod 3 has been provided above the piston with the constriction 75. In the space 74 encircling the piston rod 3 there is high pressure. In the connection 9 and also above the piston 7 there is low pressure. As soon as the constriction comes into a position in which through the constriction 75 the space 74 will [be connected with the cylinder space above the piston and accordingly through the connection 9 with the discharge, the pressure in the space 7 4, will fall and accordingly will fall in the connection 23 and the conduit 79' of the control circuit by means of which through the conduit 80 of the blocking slide 45 the actuator 46 of the pilot slide was kept under high pressure. The throttle 76 momentarily prevents the high pressure in the conduit 77 from penetrating into the conduit 79. The pressure drop in the actuator 46 has the result that the spring 48 will move the pilot slide 47 back into the shown position as a result of which the control slide 29 is switched back and the conduits 30, 32 and connection 9 are again brought under high pressure, while the conduit 34, 33 andthe connection 10 are again connected with the discharge. Before the piston 7 has reached its extreme upper position a pressure rise already takes place in the space above the piston and in the pressure accumulator 63 which pressure together with the high pressure delivered by the pump assure that the piston is returned with great energy to perform its next working stroke. As the result of the pressure rise in the conduit 60 and accordingly in the actuator 49 of the slide 45 this slide is returned in the position shown in which the port 23 of the control circuit is put out of operation so that the pressure in 23, 74, 78 and 79 may gradually increase thrcrugh the throttle 76 without any objection. It will the clear that as a result of the switching of the control slide 29 back into the shown position the pressure drop in conduit 33 will also exist in the conduit 62 so that the overpressure valve 61 will operate accordingly and will stimulate the pressure rise in conduit 32 and if present also in the high pressure accumulator 63 so that already during this period of switching at the end of the return stroke the energy available in stopping the piston is partly accumulated in the high pressure accumulator 63 and may be used during the next working stroke.

With a device having a plurality of pistons different stroke heights may be obtained by providing different heights for the constriction of the respective piston rods and providing a switching member by means of which at choice, the connections 22, 23 of the respective cylinders with the blocking slide 45 can the defined.

It is observed that the high pressure accumulator 63 is not always necessary but under certain circumstances promotes a smooth operation and a high efficiency.

The control of the forces which are generated enables mounting the pump and its driving mechanism on the device, e.g., on the housing. This makes the construction very compact. The pump may 'be a pump with a constant output. It, however, is also possible to use a pump with a variable output, e.g., a pump which automatically adapts its output to the requirements.

FIG. 3 shows a simplified diagram of a modification of the diagram of FIG. 2. It does not show a high pressure accumulator nor a low pressure accumulator. Also the one way valve 31 with overpressure valve 61 and control conduit 62 of FIG. 2 are left out.

The main difference with respect to FIG. 2 is that the control at the end of the return stroke is different. The operation is as follows. During the working stroke as shown liquid under pressure is supplied by the pump 124 through conduit 126 and the shown position of the control slide 129 into the main conduit 132 and the connection 109 into the cylinder space 106 and drives the piston 107 downwardly. At the end of the working stroke the piston 107 passes the port 122 and high pressure acts through conduit 143, blocking slide .145 (in the position shown), conduit 181 upon the actuator 146 of the pilot slide 147. This slide will change its position and take the one shown in the upper half of the slide 147, which movement due to the pressure in the actuator 146 takes place against the spring 148 as a result of which the actuator 157 of the control slide 129 will be connected with the discharge 136 through conduit 160 so that the spring 156 will move the slide 129 to the position shown in the upper half of the slide. Due to this .the conduit 132 will be connected with the discharge and the conduit .133 with the high pressure supply. The piston returns to perform its return stroke. The blocking slide in the same way as shown in FIG. 2 takes another position due to the pressure drop in conduit 132 and accordingly in conduit in which position the pressure in 179 through 181 into 146 will keep the pilot valve 147 in its switched position.

In this diagram an additional slide has been provided with two actuators 191, 192 and a spring 193. This slide 190 is placed in a connection between the conduit 179 and the discharge .160 indicated with the reference 194. The slide may close or open this connection. The actuator 191 is connected through the conduit 195 with the port 123. The spring 193 urges the slide into the position shown. The actuator 192 acts in the same direction.

If the piston returns and during the return stroke passes the port 123 the high pressure existing below the piston 107 will enter the conduit 195 and accordingly the actuator 191 and will place the additional slide 190 in the position shown in the lower half of the slide in which the connection with the discharge is open. This means a pressure drop in the actuator 146 of the pilot slide 147 which accordingly will move and will actuate the switching of the control slide 129. The throttle 176 in the same way as shown in FIG. 2 will throttle and temporarily isolate the high pressure in the conduit 152 allowing the pressure drop in 179 and accordingly in the actuator 146.

As soon as the control slide 129 has been switched the conduit 132 again obtains high pressure and the piston will again perform its downward stroke. Due to the connection 196 the actuator 192 together with the spring 193 will assure that the additional slide 190 has resumed its original position. The spring 193 will take care of this movement of the slide 190 notwithstanding the fact that there is still high pressure in the actuator 19 1 which high pressure during the reversal of the movement at the end of the return stroke shortly will drop but again will rise as soon as the downwardly moving piston 107 has passed the port 123. Both actuators 191 and 192 then will have the same pressure but the spring 193 takes care that at any rate the slide 190' is in the correct position.

FIG. 4 is a time-impact diagram in which P is the force necessary to overcome the resistance of the soil and in which P is the force which may not be passed to prevent damage of the pile. This diagram occurs if the member between the ram and the pile head is a normally resilient member. From the diagram it follows that during the time to t the energy transmitted to the pile is necessary for building up the force to overcome the resistance of the soil. This is lost energy. Also the period from 1 to t, is useless since then the force is smaller than the resistance of the soil. The maximum force has reached in a short period between t and t with the result that with relatively short piles high tensional forces may occur in the piles which may break the pile.

FIG. 5 shows what happens if according to the invention a prestressed resilient member is used. In this graph P again is the minimum force level necessary for overcoming the resistance of the soil. P again is the critical limit below which the maximum value PB should be kept. From the diagram it appears that the minimum force P is achieved within a shorter time and is maintained almost up to the end of the impact period. Accordingly there is a much smaller loss of energy. It further appears that the maximum impact is only obtained after a relatively long period t which means that the risk of tensional forces will occur only in longer piles and only relative to a much smaller value of PB-PA. Due to the fact that it is possible with the embodiment according to the invention to control the magnitude of prestressing and accordingly to obtain any desired time-impact diagram it is possible to obtain the most efiicient pile-driving device in combination with the exact control and constant level of the acceleration during the working stroke.

I claim:

1. A hydraulic pile-driving device comprising: a ram vertically guided in a housing which can be placed on top of a pile, at least one cylinder with a piston, the cylinder forming a part of the housing while the piston is connected with the ram through a piston rod, 9. hydraulic circuit including a main circuit having a high pressure conduit, a source of liquid under pressure and a return conduit, a control slide with actuators, the control slide connected in the main circuit to establish fluid communication with opposite ends of the cylinder with the high pressure conduit and the return conduit in alternate fashion, an additional control circuit branching from the main circuit and having a pilot slide with an actuator,

the pilot slide selectively establishing fluid communication between the actuators of the control slide and the high pressure conduit and the return conduit, the pilot slide actuators selectively connectable through a blocking slide with a port located adjacent the lower end of the cylinder and with the high pressure conduit through a throttle restriction, a blocking slide actuator actuated by pressure in high pressure conduit and return conduit, the blocking slide having passages and connections such that in one position it connects the actuator of .the pilot slide with the cylinder port adjacent but spaced from the lower end of the cylinder, and in its other position connects the same actuator with the throttle restriction in the high pressure conduit.

2. A hydraulic pile-driving device as in claim 1 wherein the connection between the blocking slide and throttle restriction is also connected with an annular space at the upper end of the cylinder, which space encircles the piston rod and is separated from the cylinder space and positioned above the connection of the cylinder with the main circuit, and further comprising a reduced diameter constriction on the piston rod above the piston for allowing the annular space to be connected with the cylinder space above the piston when the piston is near the upper end of its stroke.

3. A hydraulic pile-driving device as in claim 1 wherein the connection between the blocking slide and the throttle restriction has a connection with the return conduit in which an additional slide is provided for selectively opening and closing this connection by means of an actuator operating against spring pressure and connected with a cylinder port located below the upper end of the cylinder, the additional slide and connections constructed such that upon pressure supplied to the actuator of the additional slide the additional slide opens the connection with the return conduit, while the spring of the additional slide is assisted by a further hydraulic actuator, the further hydraulic actuator acting in the direction of the spring and being connected with the main circuit to the upper end of the ram cylinder.

4. A hydraulic pile-driving device as in claim 2 wherein there are a plurality of cylinders each with a piston and piston rod, the reduced diameter constrictions of the piston rods of the respective pistons having diiferent lengths, and a switch member provided for selectively connecting one of said plurality of cylinders with the blocking slide.

5. A hydraulic pile-driving device as in claim 1 further comprising a low pre sure accumulator in the return conduit of the main hydraulic circuit, and a one way valve connecting the accumulator with the portion of the return conduit of the main hydraulic circuit which extends from the control slide to the lower end of the cylinder.

6. A hydraulic pile-driving device as in claim 1 wherein a conduit of the main hydraulic circuit extending between the control slide and the upper end of the ram cylinder is provided with a high pressure accumulator, a one way valve, and an over pressure valve connected in parallel with the one way valve, the over pressure valve and one way valve located between the high presure accumulator and the, control slide, the over pressure valve :being connected through a control conduit with the return conduit portion extending between the control slide and the lower end of the cylinder, the over pressure valve having passages in connections such that with high pressure -in its control circuit it provides for free passage While with low pressure in its control circuit it operates as an over pressure valve.

7. A hydraulic pile-driving device as in claim 6 wherein the hydraulic circuit includes a low pressure accumulator in the return conduit, and a one way valve connecting the low pressure accumulator with the portion of the return conduit which extends from the control slide to the lower end of the ram cylinder.

8. A hydraulic pile-driving device as in claim 1 further comprising a member positioned between the ram cylinder 1 1 and the top of a pile which member is resiliently prestressed in the direction of the working stroke of the ram cylinder.

9. A hydraulic pile-driving device as in claim 8 wherein the resiliently prestressed member comprises a hammer body vertically shiftable within a further cylinder in the lower end of the ram, the hammer 'body being closed at the bottom and open at the top, a piston in fluid tight shiftable engagement with the cylindrical internal Wall of the hammer body, a space enclosing gas under pressure below the piston and the bottom of the hammer body, and aconnection from a space above the hammer connected with the main hydraulic circuit, the connection established by a conduit with a one way valve, the conduit extending through the piston rod of the ram cylinder and having one end opening into the cylinder space above the piston and the other end opening into the space above the hammer body.

References Cited UNITED STATES PATENTS 878,645 2/1908 Lindsay 173134 X 2,396,627 3/1946 Wohlmeyer 173134 X 2,755,783 7/1956 Kupka 173-134 X 3,157,070 11/1964 Heidrich 173--134 X NILE C. BYERS, IR., Primary Examiner. 

